Smart monitoring system for tilted cap position on PET bottles

Economical smart camera from Leuze electronic ensures process reliability during the detection of cap position

Cap monitoring in the beverage filling industry is a very challenging task for which very intricate inspection systems are typically used. Voigt Technology has developed a new, innovative concept for Aqua Römer based on an LSIS 412i smart camera from Leuze electronic. It is characterized by considerably lower investment costs and simple handling. Furthermore, its detection performance is brilliant.

"Applying plastic screw caps onto PET bottles during beverage filling involves certain difficulties which make the subsequent monitoring of the screw cap for presence and correct position very demanding," explains Bernhard Voigt. He, a service technician with decades of career experience, is a sought-after specialist in the global beverage industry.

Voigt Technology e. K. gives advice in all areas of quality control, provides services and develops solutions for special requirements. One of the latest projects is a brand new, innovative concept for the monitoring of twist caps on PET bottles based on an LSIS 412i smart camera from Leuze electronic.

High demands in the beverage industry

In broad terms, Voigt lists bottle tolerance and environmental conditions as the main causes for the tough demands on such solutions, coupled with high process speeds typical in these applications. Particularly after several washes, PET bottles exhibit considerable tolerances in height which can easily equal several millimeters. In addition, shrinkage is not necessarily symmetrical due to thermal load in the washing plants. This means that skewing can occur to a certain extent on the bottle neck or the entire bottle.

Moreover, the filling liquid, such as mineral water with high or low pressure, contributes to the height and width tolerances of the PET bottles. For instance, spraying contamination, meaning drops of moisture on the bottle exterior, can make the detection of contours more difficult. Due to system-related factors, high speeds and mechanical conveyor conditions – from belt unevenness to guide rail adjustment – contribute to bottles not being optimally aligned when they pass by a detection system. In spite of very fast and exact triggering, the bottles wobble or move during recording. At a high throughput of 30,000 to 40,000 bottles an hour, the range of these movements can be considerable.

Generally, very elaborate systems are used for such tasks which work with multiple detection technologies (X-rays, ultrasonics or image processing systems). Aqua Römer, one of the largest bottling plants for mineral water in Baden-Württemberg, also utilizes several inspection systems. In light of this, Bernhard Voigt was given the job of finding a more economical and easier-to-use alternative. At the same time, the main goal was to improve detection quality in order to reduce the pseudo-scrap rates caused by redundancy.

Everything on board

The solution is based on a LSIS 412i smart camera from Leuze electronic which uses a transmitted light source created specifically for the application. "One big advantage of the LSIS 412i is that is unites all necessary components from image processing to data memory and display to interfaces in one device," states Voigt, who particularly appreciates that no additional connector units are necessary. The robust industrial version with metal housing and the fixtures selected in this case including scratch resistant protective glass also fulfill the requirements of the beverage filling industry in accordance with protection rating IP67.

With pixel-perfect image processing, the LSIS 412i performs presence inspections and monitoring for tilted cap position beautifully. Using the so-called BLOB (binary large object) analysis, it's possible to react to tolerance factors of PET bottles in a particularly smart way and at the same time to detect caps tilted in any direction, 360° around the neck of the bottle, with only one camera.

With tracked test ranges

BLOBs are contiguous areas of pixels whose light intensities lie between defined boundaries. By restricting BLOB features such as length, width, height, surface, shape factor or circumference, objects or object groups can be reliably detected and differentiated on the basis of their geometric features – even when other processes are supplying incorrect results.

"The high process reliability when monitoring for tilted cap position is also largely based on the option of tracking test ranges," adds Voigt. The bottle cap is "searched" both horizontally and vertically with additionally-created test fields in order to then track the actual test fields accordingly. This effectively compensates position shifts and height tolerances for monitoring. For testing, the support ring in the neck area is ultimately used as a reference. The distances on both sides of the support ring to the retaining ring under the cap as well as height and width data of the cap provide information about whether its position is correct.

Smart camera solution in detail

The solution created by Voigt Technology consists largely of the LSIS 412i smart camera which is equipped with an opposing transmitted light source instead of its own source of illumination. An additional computer unit is unnecessary and configuration software does not have to be installed. Program memory and image processing are already integrated in the LSIS 412i camera.

Configuration is performed using the webConfig configuration interface directly via the web browser. A service Ethernet interface enables quick and easy access to the device via a laptop for commissioning. The already present process interfaces, Ethernet, RS 232 and eight freely-configurable digital I/O ports, make for low integration costs.

A retro-reflective photoelectric sensor with polarization filter (PRK), also from Leuze electronic, is used for triggering. It enables the reliable detection of transparent media and, with its zinc diecast housing and glass cover, it is robust enough for demanding environments. With a switching frequency of 1000 Hz and a response time of 0.5 ms, it is ideal for triggering mineral water bottles. The downstream diverter is directly controlled via an LSIS 412i output signal.

"All in all, the new monitoring system for tilted cap position is considerably more economical and easier to use than conventional solutions. In addition, it does not require setting or any maintenance worth mentioning by the user. Furthermore, the detection rate is an excellent 99.8%," sums up Voigt.